Patent GB-1375091-A (GB)
IPC: G01B9/02
Filed: 1972-01-03
Abstract: 1375091 Measuring the distance between two objects with an interferometer HEWLETTPACKARD CO 3 Jan 1972 [1 Feb 1971]  136/72 Heading G1A [Also in Divisions G2 and G4] In.an interferometer measuring system for measuring the distance between two members 18, 19 Fig. 2 errors due to external conditions affecting the distance between the components of the interferometer and the members 18, 19 are reduced by mounting a beam splitter on one of the members together with the reflector for light, the path length of which is fixed, and mounting the reflector for light, the path length of which is varied, on the other of the members. In the embodiment of Fig. 2 a light polarizing beam splitter 35 and fixed light reflector 37 are mounted on member 19 and a light reflector 38 is mounted on the other member 18, the arrangement being such that light from an externally mounted laser 12 is split at the beam splitter 35 into two orthogonally polarized components one being directed along the fixed path to the reflector 37 and the other being directed along the variable path to the reflector 38. The reflected light beams are recombined by the beam splitter 35 and directed to a further polarizing beam splitter 39 which separates the components into two orthogonal polarization planes for transmission to two photo-detectors 32, 33 which with associated circuitry derive a count representing the displacement between the two members 18, 19. In the embodiment of Fig. 3 the laser 41 provides a two frequency beam output f 1  and f 2  which is separated into two orthogonal components by quarter wave plate 42 and half wave plate 43. A beam splitter 44 directs a portion of the two beams to (1) a reference voltage detector 46 giving a signal f2-f1 to one input of a reversible counter 47 and (2) a polarizing beam splitter 48 which directs the component of frequency f2 to fixed reflector 49 and the component of frequency f 1  movable reflector 51. The reflected beams are recombined and directed via a demodulating polarizer 52, to a photo-detector 53 giving an output of frequency f 2 -f 1  Œ #f to the other input of the reversible counter 47 so that a count representing #f is derived. In a further embodiment Fig 4 (not shown), two distance measurements are made simultaneously. One measurement is effected by directing one component of the laser beam from a beam splitter (57) to a polarizing beam splitter (61) directing light to a fixed and movable reflector (62, 63) as in the embodiment of Fig. 3. The beam splitter (57) also directs light onto a further polarizing beam splitter (65) directing light to a fixed and movable reflector (66, 67) respectively. The reflected light is recombined at the polarizing beam splitter (65) and directed via the beam splitter (57) to a further photo-detector (68). The pitch of one movable member relative to another may be measured by directing light from a beam splitter (71) Fig. 5 (not shown), on to a reflector (52) attached to one of the members via a mirror (74) on to a reflector (73) attached to the second member so that changes in the value X1-X2 are measured. If the second reflector is mounted immediately above the beam splitter changes in the value X1-Y may be measured.